1. Field of the Invention
This invention relates to a matching circuit, which transforms, for example, an impedance of a source circuit of the balanced type (or the unbalanced type) into that of a load circuit of the unbalanced type (or the balanced type), while transforming a balanced (or an unbalanced) signal into an unbalanced (or a balanced) signal.
2. Description of the Related Art
The high frequency power amplifier for a medium- or short-wave radio broadcasting solid-state transmitter has generally employed the class D/bridge circuit system, because it must handle a large high frequency power. In designing the amplifier, semiconductor elements such as field effect transistors (FETs), are usually used for its amplifying o active elements. With the limited tolerable power and the breakdown voltage of the elements, a balanced circuit system is employed for the amplifier circuit. On the other hand, a load circuit including a transmission antenna is of the unbalanced type, which is coupled for reception with the output of the amplifier. To match the impedance and the circuit system of the source and load circuits, a matching circuit has been inserted between those circuits. The matching circuit inserted must satisfy the following conditions:
(1) When the power source is grounded at one end, the circuit system including those source and load circuits is frequently designed so that the amplifier output is balanced with respect to ground, but the load input is unbalanced with respect to the same. Therefore, the matching circuit must match the balanced output of the amplifier and the unbalanced input of the load circuit.
(2) The output voltage of the amplifier, which operates in a class D mode, is rectangular in waveform. The rectangular wave voltage contains much harmonic components, in addition to the fundamental frequency components. Only the fundamental component is required for the signal to be transmitted to the load. To select only the fundamental component, the matching circuit must have a filter function, i.e., a band-pass or lowpass filtering. That is, the matching circuit must exhibit a low impedance for the fundamental component of the amplifier output, while exhibiting a high impedance for the harmonic components of the output current.
(3) In design, it is very difficult to match the input impedance of the load circuit with the optimum load impedance of the amplifier. Therefore, the matching circuit must match the amplifier output impedance with the load input impedance.
A prior matching circuit satisfying the above requirements is shown in FIG. 1.
In FIG. 1, high frequency power amplifier 11 is coupled through matching circuit 12 to load circuit 13. The matching circuit has a pair of input terminals T1 and T2, and an output terminal T3. Power amplifier 11 is made up of amplifying elements Q1 to Q4 connected in a bridge fashion between VDD power and ground lines. The junction points between elements Q1 and Q2, and between elements Q3 and Q4 are the output terminals of the amplifier. In operation, paired elements Q1 and Q4, and paired elements Q2 and Q3 are alternately operated. Thus, the output of power amplifier 11 is electrically balanced with respect to ground. Load circuit 13 with one end grounded is electrically unbalanced with respect to ground, as shown. In matching circuit 12, transformer 121 contained therein has a primary winding whose both ends are connected through input terminals T1 and T2 to the output terminals of the power amplifier. The secondary winding of the transformer is grounded at one end, and connected at the other end to the input terminal of load circuit 14, via LC bandpass filter 122 and output terminal T2.
In matching circuit 12 thus arranged, transformer 121 transforms the amplifier output impedance into the impedance of the load input impedance, and further transforms the balanced amplifier output into the unbalanced load input. To effect the impedance transformation, a turn ratio of the primary winding to the secondary winding of transformer 121 is properly selected. The rectangular wave signal induced into the secondary winding is filtered by the LC bandpass filter 122. This filter allows passage of only the fundamental frequency component contained in the rectangular wave signal, while rejecting the remaining frequency components.
The above arrangement of the matching circuit involves the following problems, however. The power amplified signal by amplifier 11 is a rectangular wave signal. Therefore, transformer 121 must handle the rectangular wave signal at low loss. As known, the rectangular wave signal contains much harmonic components, in addition to the significant fundamental frequency component. To handle this much-harmonics-contained signal at a low loss, the transformer must have a broad band frequency characteristic. Further, the power loss by the transformer, which appears as transformer heating, increases more as the frequency of the handled signal is higher. Therefore, as the fundamental frequency used is higher, the design of the transformer for it is more difficult. In the case of the transformer for handling such a high fundamental frequency, the number of turns of the windings must be small. The necessary turn ratio is also based on the small number of turns. This makes it difficult to obtain an optimum turn ratio for the impedance matching.
Thus, in the prior matching circuit, the transformer design is difficult particularly in high frequencies of the significant fundamental components. To the extreme, practically usable transformers cannot be designed.